Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Does the WMAP immage show the edge of the universe?

  1. May 14, 2008 #1
    Does the WMAP image show the edge of the universe, the edge of observable matter, both, or niether?

    Does the very even emission detection in all directions suggest that we are very near the center of the observeable universe?

    Am I correct in understanding that the immage is emitted from the surface of last scattering in our early universe (350,000 yrs after big bang)?

    - If this is correct, how did this material get 20 billion (or what ever it is)
    light years out in 350,000 years?

    - ...and how is it that we can see the immage of the original stuff, and the all
    that it has generated from the same location at the same time?
  2. jcsd
  3. May 14, 2008 #2
    Neither. If there is an edge to the universe, it is beyond the range that WMAP shows.

    No. It suggests only that, if there is an edge (which there may not be), we are too far away to see it.

    Yes it is from the surface of last scattering.

    According to the Friedmann equations, the expansion rate would have been enormously high in the early years after inflation ended, falling off with time. At the end of inflation the radius of the currently observable universe (measured at its horizon) would have exceeded the speed of light. So the universe expanded very fast during its early era.

    The CMB radiation was emitted at the time of last scattering. It has been travelling from everywhere in the universe to everywhere else in the universe since then. The particular CMB photons we receive now were emitted far enough away and long enough ago that they are exactly reaching us now. Photons that were emitted earlier or closer to us have already passed us, etc.

    Last edited: May 14, 2008
  4. May 15, 2008 #3

    Thank you for your time. I appreciate the response. I have a few follow up questions.

    I can see how the edge of the universe must be beyond the CMB but it seems like the CMB would be the edge of the observable matter.?
    - From the sources that I have read it seems to be suggested that the CMB
    photons are coming from a specific location and that they originated from a
    location further back in space than anywhere else that we see photons
    orignating from.

    If the CMB is the edge of observable matter and it is so incredibly even wouldn't we have to be very near the center of all the material transmitting it? If we were closer to one end wouldn't that background radiation be more intense in the image?

    It seems like some source point to very tiny fluctuations in the radiation intensity in the CMB immages and call them seeds of galaxies. If this is true those seeds would have formed into galaxies on our side of the CMB. They would be closer to us. They would be galaxies that we can see.? The material that would form a galaxy being observed through one insturment and the galaxy it formed being observed through another insurment in different location able to be viewed in the same night.? The following site from the university of oregon seems to be saying some of this.
  5. May 15, 2008 #4
    Hi big prairie, I'm travelling so a quick response:

    I didn't say there is an edge to the universe, I just said it's a possibility.

    The CMB photons we see now have travelled to us a specific, finite distance from points in all directions which form a sphere around us. If there is an edge to the universe, it is assuredly beyond this CMB observable sphere, because the CMB is so isotropic. The odds are it would probably be far, far beyond the CMB distance, and the universe may also be infinite.

    I'll read the site you mentioned when I return. But my understanding of the surface of last scattering is that essentially the universe is opaque to most forms of radiation beyond that point, so we can't see anything. Someday we may be able to detect neutrinos or other particles that could perhapse extend our view beyond that surface.

  6. May 15, 2008 #5
  7. May 15, 2008 #6

    Thank you.

    Yes I see what you are saying about the edge of the universe.

    If we are in the center of the CMB and we are seeing that radiation arrive from its point of origin and if it is coming from material ejected out by a Big Bang; it seems that we must be relatively close to where the big bang started from?
  8. May 15, 2008 #7
    As I understand it (and might be wrong on one or more points), WMAP shows us what the universe looked like during the phase transition of the universe into cooler matter (electrons bound to protons and neutrons) and released a streaming set of photons (the CMB). The emission was in all directions because (hopefully) the universe expanded in all directions, and still is.

    WMAP is not an unidirectional picture, which may be counterintuitive when examining the extrapolations. Rather, it is a panoramic projection from all directions (since the universe expanded in all directions). I think you're right, the time of coupling was approx 350,000 years. The CMB has been free streaming ever since that time.

    I'm relatively sure that WMAP displays the entire universe at the time of decoupling as we measure it. The "edge of the universe" could be described as the furthest observable point in our universe currently (as the universe is expanding faster than light can traverse the distance between far off galactic systems).
  9. May 15, 2008 #8

    Thank you for your response.

    I believe you are right about the CMB immages that we see being a panorama type view. As we look out from the earth we see the CMB comming from every direction. And, I think you are right in saying that it seems as if the inital event would have sent EMR in every direction. Of course we only get to see the CMB that was emitted back in our direction. Apparently that is the EMR we capture to make the images of the CMB.

    Some internet sites say that we can see irregularities in the very fine detail images of the CMB that are the seeds of galaxies. If I am understanding what is being said about the topic I am struggling understanding how it comes together. The implication is that these seeds formed the galaxies between here and there.? How? A seed comes before the tree not after. It seems like all of the stuff produced by that CMB generating event would be on the otherside of where todays CMB originated from. Perhaps the "galaxy seeds" is refering to galaxies yet to be formed that direction from the CMB and not what we see this side but that is not how it sounds in what I have read.
    We see the material today where it was 15 b.y. ago. We are also supposed to be seeing it the way it was 350,000 yrs after a Big Bang event. If it took the initial material that far and that long to coalesce into matter what happened to all of the material this side of that event? If we were in our position in space 14by ago would we see a CMB 14by out? How aboout if we were here looking out 13by ago would we see the CMB 2by out? Didin't the stuff have to get that far out and that spread out before it could cool enough to coalesce into a somewhat familiar form of matter? If so how did our matter (apparently smoewhere near ground zero) coalesce?
  10. May 15, 2008 #9


    User Avatar
    Science Advisor

    When we say that the fluctuations in the CMB are what seeded galaxies we aren't referring to specific fluctuations and galaxies. You can't say 'see that hotter part of the CMB, that is what formed that galaxy'. What we mean is that the fluctuations present in the early Universe, when the CMB photons were last scattered, are what seeded galaxies to eventually form. But of course the galaxies formed from the regions of the Universe that scattered the CMB photons that we are seeing now will not be visible for several Billion years, since the light emitted in the early Universe has only just got to us, so the light that was emitted from the first galaxies to form in that region is still on its way.

    Looking back to higher and higher redshifts is like viewing the life of the Universe at earlier and earlier times.
  11. May 16, 2008 #10

    Thank you.

    From what you have said it sounds as if we are on the young side of the CMB and that it is moving out from where it started. This would make sense to me. It is the way we would expect the thing to work.

    As I read what NASA has to say,however, it sounds as if we can't see throught the CMB to an earlier time because the CMB is the surface of last scattering. From what I understand it is not that there isn't EMR bouncing around on the other side but just that it is last scattered from the region of the CMB and starts heading our way. This would imply that the earlier time is the other side of the thing (CMB). Shouldn't the earlier time be on our side? See the quote below.

    Credit: NASA/WMAP Science TeamA representation of the evolution of the universe over 13.7 billion years. The far left depicts the earliest moment we can now probe, when a period of "inflation" produced a burst of exponential growth in the universe.

    (the diagram NASA reffers to showed the CMB far left and the probe(outr location) far right)

    Please look at he diagram posted here.


    another quote

    The novel feature of these polarization observations is that they reveal directly the seeds of galaxy clusters and their motions as they proceeded to form the first clusters of galaxies.

    another quote

    The expanding universe cooled and by 400,000 years after the Big Bang it was cool enough for electrons and protons to combine to form atoms. Prior to this time photons could not travel far before colliding with an electron, and the universe was like a dense fog, but at this point the universe became transparent and since that time the photons have streamed freely across the universe to reach our telescopes today

    last two quotes from physlink.com/news/101004GalaxySeeds.cfm

    Do you see where i am getting this line of thinking from?
  12. May 16, 2008 #11
    I have a general CMB question for all interested that I think relates to this discussion. Compare the CMB that we observe today to the CMB that we observed one year ago. Assuming CMB travels at light speed, shouldn't the CMB 'map' that we draw of the cosmos today be an image of the cosmos one light year further away from us in all directions? And therefore, wouldn't it be probable that the contours of this map and the inhomogeneities be different as a result? Or if one year's difference in observation is negligible, wouldn't the same be true for the difference of a thousand years, or a million years, which is still a very small figure compared to the 13 billion year origin of the signal?
  13. May 16, 2008 #12


    User Avatar
    Gold Member

    Very crude calculation:
    The characteristic scale of the bumps in the CMB is about half a degree or about 1%.
    So they should change completely in about 1% of 13 billion years or in 130 million years.
    The change in one million years should be of the order of one percent.
    The change in shorter periods effectively unnoticeable.
    Jim Graber
  14. May 16, 2008 #13

    Would you happen to have any idea how much closer to our current location material would have been able to get and still be able to coalesce? It seems atoms must have come together everywhere that we can see but wouldn't the energies have been to high this side of 15by out? If it eventually spread out and cooled after 350,000 yrs then wan't the coalescing event working its way back this direction from the location of the CMB?
  15. May 17, 2008 #14


    User Avatar
    Gold Member

    big prairie,
    Sorry, I don't understand your questions.
    If by coalescence you mean recombination, it happened simultaneously everywhere 350,000 years after the big bang.
    If by the location of the CMB, you mean the surface of last scattering we currently see, it appears to expand by one light year per year, but we are actually seeing different material.
    Hope this helps.
    Jim Graber
  16. May 17, 2008 #15
    Any thoughts as to whether the CMB will eventually die out in the future? 13 billion years? 130 billion years? Or is it physically impossible to ever be at a vantage point where you will not be observing the radiation from the time of decoupling?
  17. May 17, 2008 #16
    Hi All - seems like this is tied to the open/closed universe question. If it expands forever (open) seems like the CMB will continually be further redshifted and diluted. If it begins to collapse (closed) seems like CMB would be blue shifted and more concentrated in smaller space. ... this is probably oversimplified ....
  18. May 17, 2008 #17


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    The Return of a Static Universe and the End of Cosmology
    Authors: Lawrence M. Krauss (1,2), Robert J. Scherrer (2) ((1) Case Western Reserve University, (2) Vanderbilt University)
    (Submitted on 2 Apr 2007 (v1), last revised 27 Jun 2007 (this version, v3))

    Abstract: We demonstrate that as we extrapolate the current LambdaCDM universe forward in time, all evidence of the Hubble expansion will disappear, so that observers in our "island universe" will be fundamentally incapable of determining the true nature of the universe, including the existence of the highly dominant vacuum energy, the existence of the CMB, and the primordial origin of light elements. With these pillars of the modern Big Bang gone, this epoch will mark the end of cosmology and the return of a static universe. In this sense, the coordinate system appropriate for future observers will perhaps fittingly resemble the static coordinate system in which the de Sitter universe was first presented.

    Comments: 5th prize 2007 Gravity Research Foundation Essay Competition, published in General Relativity and Gravitation October 2007;

    the consensus view is the U keeps on expanding

    after it has expanded another 1000-fold, the temperature of the CMB will instead of being 2.7 kelvin be
    2.7 millikelvin. the microwaves will be too weak for anybody to detect

    the other evidence from which our cosmology model has been deduced will also fade away

    now is a great time to learn about the universe
    future people will not have it so easy, they will lack the observational data we have
    Last edited: May 17, 2008
  19. May 17, 2008 #18


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    you are right, except that currently that question is considered closed
    in the standard model of cosmology the universe is destined to continue expanding indefinitely
    (it may be spatially finite or spatially infinite, we do not yet know, but because of the cosmological constant that is irrelevant----it can never start to collapse)

    there are nonstandard cosmologies in which the universe starts to collapse eventually but ordinary working cosmologists don't consider them or study them, as a rule, because the evidence seems to rule them out

    however YOU ARE RIGHT that if distances started to contract globally then the the CMB would be less and less redshifted, until after a 1000 fold contraction it would again be its original temperature of about 3000 kelvin that it was when the light was released.

    at that point it would not be redshifted at all and the sky would be glowing red hot

    and then, with further contraction, the CMB would be BLUEshifted and it would be hotter than 3000 kelvin.

    the redshift of the CMB is always roughly proportional to the factor by which distances have expanded since the time the CMB photons were released, at present that expansion factor is about 1100 (or roughly 1000)
    distance has increased by a factor of about 1000 so the redshift of the wavelengths is also ab out a factor of 1000

    there is no evidence that it will ever contract, but if it started to constract then this factor would slowly decline until after a long time it would be 1, and there would be no CMB redshift at all-----that is the 3000 kelvin sky point.
  20. May 18, 2008 #19
    Thanks Marcus for bringing me up to speed, I guess I've been out of touch. The cosmological constant: Einstein's 'big mistake' .... do we know yet whether the expansion will continue at an accelerating rate? Does this acceleration have anything to do with the inflation that happened very early?
  21. May 18, 2008 #20


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    There are several people around who are professionals and can speak confidently about the current state of knowledge. I'm a science watcher (not a working cosmologist) just intensely interested in these things.

    My impression is that cosmology has a really good model, the LambdaCDM, that more or less everybody in the field uses, and which fits the data remarkably well. The more data that comes in the better it looks.

    But LambdaCDM has large unexplained elements. so there is a lot to work on.

    So my impression is that WE DON'T KNOW if expansion will continue as you asked about. We have an excellent good-fit model that says it will! but we don't fully understand all the features of that model, and we can't guarantee that the model won't eventually be replaced by something else. If you believe the model, well then sure, expansion will continue to accelerate (but will not go to big rip levels). Larry Krauss has a good thumbnail sketch of the future, based on assuming the consensus model.

    You also asked about the conjectured inflation scenarios. Inflation scenarios were invented to explain some puzzling observations. and some kind of inflation in the early universe is widely accepted. But there is a big BUT. There is no one scenario everybody agrees to. there is no known mechanism. The scenarios are still conjecture. there may even be alternative ways to resolve the puzzles without resorting to inflation. it is still iffy.

    Given that, we can still draw similiarities between the imagined mechanism of early universe inflation (the conjectured scalar field called the "inflaton" that nobody has actually ever seen) and the "dark energy" which people often imagine is the source field for the much more slowly accelerating expansion we measure now.

    Although the sizes of the effects and the implied energy densities are different, there is a striking similarity. I haven't read anybody claiming to have proven that they are actually the same thing, but as you have noticed there is a kind of parallelism.

    Cosmology involves a lot of solid science with a lot of uncertainty and unknowns. I respect cosmologists for being able to cope with that kind of situation and function well with the incomplete information they have. I don't demand complete answers to all the big questions from them (not right away at least :biggrin:). All you can do is take it as it comes. The field is advancing well. Year by year they find out more and the uncertainty gets reduced a little.
    Last edited: May 18, 2008
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook